1. Field of the Invention
The present invention relates generally to processes for semiconductor manufacturing and more particularly to characterizing and monitoring lithographic projection lenses in-situ.
2. Description of the Related Art
Projection imaging systems, also referred to as lithographic projection systems, have many uses in various industries. For example, lithographic projection systems are used to produce patterns on semiconductor materials for use as integrated circuits and flat panel displays. The performance of lithographic projection systems greatly influences the manufacturability and cost of manufacturing semiconductor chips and flat panel displays.
In general, the performance of lithograph projection systems is limited because of aberrations, which are the deviation of a projection lens' performance from a “perfect” lens or from the diffraction limit. As the resolution required from lithography projection systems increases, for example as low as 100 nm and below, the ability to measure the state of the optical aberration of projection lenses becomes increasingly important. For example, aberrations as small as 10 milliwave (mλ) or less can cause significant shifts and distortions in patterns.
Techniques have been developed to compensate for aberrations in lithography projection systems. For example, distortion and field curvature data from images exposed using a lithography projection system are used to design “figured” optical surfaces that are placed in the optical path of the projection system to compensate for aberrations of projection systems. A drawback to these techniques is that they only consider distortion and field curvature aberrations. Distortion and field curvature correspond to the lowest order aberrations of an imaging system, namely field dependent tilt and focus. In order to ascertain the degree of correction and method of correction useful for higher order aberrations, data in addition to distortion and field curvature are needed.
Application of a conventional interferometer to a projection imaging system can provide high quality wavefront data. However, a drawback to use of a conventional interferometer is that it requires removing, or significantly altering or disturbing, the lens column of the lithographic projection system. Removal of the lens column can introduce uncertainties into the measurement and require significant downtime from productive operation.
Due, at least in part, to the drawbacks in using a conventional interferometer in-situ, techniques have been developed for determining distortion, field curvature, best focus, astigmatism, and the aerial image of a projection imaging system in-situ. Of these various in-situ techniques, the greatest amount of information is usually provided by in-situ aerial image measurements. However, a drawback to in-situ aerial image measurements is that the light level is generally low, leading to long exposure times or poor signal to noise ratios. In addition, the reconstruction of the aberrated wavefront is ambiguous unless several out of focus exposures are performed.
Thus, there is a need for an improved technique to measure the optical aberrations of lithography projection systems. The present invention satisfies this need.